In communication networks such as Long Term Evolution (LTE) networks (e.g., LTE E-UTRAN networks), a measurement gap length is defined for a user equipment (UE) to identify and measure signals from base stations associated with carriers other than the carrier associated with a base station currently servicing the UE. Such signals may be transmitted on a different frequency channel than a frequency channel on which the UE communicates with the base station that currently serves the UE. This process may be referred to as identifying and searching inter-frequency and/or inter-radio access technology (RAT) cells.
According to the current standard as defined in 3GPP TS 136.133 V12.7.0 (2015-03), a UE is configured with one of the two measurement gap patterns: either with measurement gap repetition periods (MGRPs) of 40 ms or with MGRPs of 80 ms. During a MGRP, for a duration equal to a measurement gap length, the UE may perform the above identifying and searching for inter-frequency and/or inter-RAT cells. The measurement gap length is set to 6 ms (i.e., 6 subframes). During the measurement gap length, the UE cannot transmit any data and is not expected to tune its receiver on any of the E-UTRAN carrier frequencies of base station currently serving the UE. Therefore, for the duration of the measurement gap length, the interruption on data transmission between the UE and the base station serving the UE is at least 6 ms out of every 40 ms or every 80 ms, depending on the measurement gap pattern configuration.
However, in reality the interruption on the data transmission experienced by the UE may extend beyond the above described measurement gap length. FIG. 1 illustrates the effective interruption in frequency division duplex (FDD) downlink data transmission between a user equipment and a base station, when the user equipment is configured with a measurement gap pattern.
As shown in FIG. 1, an assumption is made that the measurement gap length 100 starts at the subframe n for 6 ms, thus covering subframes n to n+5. For a downlink (DL) Physical Downlink Shared Channel (PDSCH) transmission at a subframe m from the base station (BS) to the UE, the UE needs to send the HARQ ACK/NACK back to the BS on the uplink (UL) subframe m+4 in order to inform the BS of whether the UE has received the DL subframe m correctly or not.
However, for DL subframes m, where m={n−4, n−3, n−2, n−1}, the corresponding UL subframes m+4 falls into one of the subframes n, n+1, n+2 and n+4, all of which fall within the measurement gap length 100. Therefore, the UE will not be able to send HARQ ACK/NACK back to the BS (indicated in FIG. 1 using dashed lines with an “X” mark on each of the dashed lines), and the BS will not know whether the UE has received the downlink (DL) subframe package correctly for subframe m={n−4, n−3, n−2, n−1}. Accordingly, in addition to subframes corresponding to the measurement gap 100, the subframes m={n−4, n−3, n−2, n−1} are not suitable for sending DL packages to the UE (shown as the shaded subframes n−4 to n+5 in FIG. 1). This effectively extends the interruption in FDD downlink data transmission from 6 subframes (6 ms) to 10 subframes (10 ms).
FIG. 2 illustrates the effective interruption in time division duplex (TDD) downlink data transmission between a user equipment and a base station, when the user equipment is configured with a fixed measurement gap pattern.
For TDD DL data transmission, the impact of the measurement gap length 200 on data transmission interruption for the UE depends on the UL/DL configuration as well as a measurement gap offset. FIG. 3A illustrates the TDD Uplink-Downlink configurations table and FIG. 3B illustrates the TDD Downlink association set table, as defined by Table 4.2-2 in 3GPP TS 136.211 V12.5.0 (2015-03) and Table 10.1.3.1-1 in 3GPP TS 136.213 V12.5.0 (2015-03), respectively.
Referring to FIGS. 3A and B, an assumption is made that the UP/DL configuration 2 is chosen. Furthermore, an assumption is made that a measurement gap offset is equal to 1. In FIG. 3A, “D” denotes a downlink subframe reserved for downlink transmissions, “U” denotes an uplink subframe reserved for uplink transmissions and “S” denotes a special subframe with the three parts: DwPTS, GP and UpPTS. DwPTS may be used for DL transmission, GP is safeguard period, and UpPTS may be used for UL transmission. As shown in FIG. 3A, for configuration 2, subframes 1 and 6 are special subframes that contains safeguard period (GP) for switching between UP and DL transmission every 5 ms.
As shown in FIG. 3B, for HARQ ACK/NACK to be sent back to the BS in subframes 2 and 7, the DL PDSCH transmission from the BS to the UE would have to take place in 4th, 6th, 7th and 8th subframe preceding subframes 2 and 7. Note that the 6th preceding subframe corresponds to a special subframe, where the DL PDSCH transmission from the BS to the UE may take place only in the field DwPTS.
Referring back to FIG. 2, assume that the measurement gap length 200 starts at subframe n(=1) for 6 subframes (6 ms) to subframe n+6(=7). According to configuration 2, the UL transmission of HARQ ACK/NACK in subframes n+1 (corresponding to the 2nd subframe discussed above) fall within the measurement gap length 200, which results in the interruption to DL PDSCH transmission in subframes n−3, n−5, n−6, and n−7.
Furthermore, 3GPP TS 36.133 V12.7.0 (2015-03) defines that if a subframe before the first subframe of the measurement gap length 200 is a DL subframe, transmission on the UL subframe immediately after the end of the measurement gap length 200 is not allowed. Thus, the UL transmission of HARQ ACK/NACK in subframes n+6 (corresponding to the 7th subframe discussed above) is not allowed, which in turn results in the interruptions to DL PDSCH transmission in subframes n−1 and n−2.
Therefore, in the TDD DL data transmission, in addition to subframes corresponding to the measurement gap 200, no DL transmission is scheduled on subframes n−7 to n−1. Accordingly, the effective interruption in DL data transmission for the UE extends from within the measurement gap length 200 of 6 subframes (n to n+5) to additionally include the 7 subframes before the start of the measurement gap length 200 (n−7 to n−1), as shown in FIG. 2.
In summary, the effective length of interruption in data transmission for the UE extends beyond the measurement gap length, according to the current state of the art.